Thyroid hormone (TH) is essential for normal brain development and may also promote recovery and neuronal regeneration after brain injury. TH acts predominantly through the nuclear receptors, TH receptor alpha (THRA) and beta (THRB). Additional factors that impact TH action in the brain include metabolism, activation of thyroxine (T4) to triiodothyronine (T3) by the enzyme 5?-deiodinase Type 2 (Dio2), inactivation by the enzyme 5-deiodinase Type 3 (Dio3) to reverse T3 (rT3), which occurs in glial cells, and uptake by the Mct8 transporter in neurons. Traumatic brain injury (TBI) is associated with inflammation, metabolic alterations and neuronal death. In clinical studies, serum levels of T4 and T3, as well as TH levels in the brain, are reduced. We have utilized rodent models of TBI to demonstrate that treatment with T4, 1 hour after injury, is protective, reduces edema, and promotes neuronal recovery. We have identified similar protective effects of TH in an in vitro model of neuronal injury from hypoxia. We will study both the mechanism of TH protection from neuronal injury as well as optimize protective treatment strategies with TH, utilizing in vivo rodent models of TBI. We have preliminary data identifying genes whose expression is impacted by hypoxic neuronal injury and those that are normalized by TH treatment. We will characterize these genes to identify specific pathways influenced by TH treatment. Hypoxic injury increases histone methylation in neurons and this is reduced by T3 treatment. We believe that this is an important mechanism for T3 protection after injury. We will also identify T3-stimulated pathways that activate neural regeneration and anti-apoptosis in neurons after hypoxic injury. We will utilize in vivo rodent models of TBI to identify the actions of TH in protection and promotion of recovery after brain injury and determine the optimal thyroid hormone treatment after TBI. We will use a mouse with global expression of a T3-reporter to determine specific brain regions with reduced T3 action after injury, as well as assess the response to systemic TH treatment after injury. We will also investigate the mechanism of TH treatment reduction in brain edema by studying the regulation of water transport. We will determine the optimal thyroid hormone preparation, dose and treatment schedule to promote brain recovery and neural regeneration in rodent models of TBI. The response to TH therapy will include assessment of the brain lesion by imaging, brain region-specific patterns of gene expression and behavioral studies. TH reduction in brain edema and cell death, and promotion of neuronal regeneration, should provide a beneficial effect after brain injury. These studies should provide guidance for clinical strategies to use TH to reduce the impact of brain injury and promote recovery. !